This invention relates to thermal inkjet printers, and more particularly to the control of the printhead firing energy.
Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes xe2x80x9cdot locationsxe2x80x9d, xe2x80x9cdot positionsxe2x80x9d, or pixelsxe2x80x9d. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting ink ejection elements. The carriage traverses over the surface of the print medium, and the ink ejection elements are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle and has a firing resistor located in the chamber. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
In an inkjet printhead the ink is fed from an ink reservoir integral to the printhead or an xe2x80x9coff-axisxe2x80x9d ink reservoir which feeds ink to the printhead via tubes connecting the printhead and reservoir. Ink is then fed to the various vaporization chambers either through an elongated hole formed in the center of the bottom of the substrate, xe2x80x9ccenter feedxe2x80x9d, or around the outer edges of the substrate, xe2x80x9cedge feed.xe2x80x9d
The ink cartridge containing the ink ejection elements is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the resistors is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Thermal inkjet printheads require an electrical drive pulse from a printer in order to eject a drop of ink. The voltage amplitude, shape and width of the pulse affect the printhead""s performance. It is desirable to operate the printhead using pulses that deliver a specified amount of energy. The energy delivered depends on the pulse characteristics (width, amplitude, shape), as well as the resistance of the printhead.
A thermal inkjet printhead requires a certain minimum energy to fire ink drops of the proper volume (herein called the turn-on energy). Turn-on energy can be different for different printhead designs, and in fact varies among different samples of a given printhead design as a result of manufacturing tolerances. In an integrated driver type printhead, the total resistance consists of the heater resistance in series with a field effect transistor and other trace resistances, each of which has an associated manufacturing tolerance. These tolerances add to the uncertainty in knowing how much energy is being delivered to any given printhead. Therefore, it is necessary to deliver more energy to the average printhead than is required to fire it (called xe2x80x9cover-energyxe2x80x9d) in order to allow for this uncertainty. As a result, thermal inkjet printers are configured to provide a fixed ink firing energy that is greater than the expected lowest turn-on energy for the printhead cartridges it can accommodate. A consideration with utilizing a fixed ink firing energy is that firing energies excessively greater than the actual turn-on energy of a particular printhead cartridge result in a shorter operating lifetime for the heater resistors and degraded print quality.
The energy applied to a firing resistor affects performance, durability and efficiency. It is well known that the firing energy must be above a certain firing threshold to cause a vapor bubble to nucleate. Above this firing threshold is a transitional range where increasing the firing energy increases the volume of ink expelled. Above this transitional range, there is a higher optimal range where drop volumes do not increase with increasing firing energy. In this optimal range above the optimal firing threshold drop volumes are stable even with moderate firing energy variations. Since, variations in drop volume cause uniformities in printed output, it is in this optimal range that printing ideally takes place. As energy levels increase in this optimal range, uniformity is not compromised, but energy is wasted and the printhead is prematurely aged due to excessive heating and ink residue build-up.
In new smart drive printheads wherein each firing resistor or each primitive does not have a dedicated connection, there may be variations due to other factors. A large number of resistors is powered by a single voltage line that receives power via an electrical contact pad between the printer electronics and the removable print cartridge. Consequently, as the data load being printed changes, the current draw through the line and the voltage as measured at the firing resistor may be undesirably varied. For instance, when many or all resistors are fired simultaneously, the print cartridge voltage may be depressed by parasitic effects, giving a lower firing voltage than when only one or a few resistors are fired.
Inkjet print cartridges can suffer from droplet ejection problems caused by formation of bubbles in the firing chamber that can cause misdirected ejection or no ejection at all. They occur when a particular nozzle has been inactive for some period of time. When a page is printed not all nozzles on a print cartridge are necessarily used. During this time of inactivity, these nozzles are often at high temperatures. Especially in pigmented ink systems, reliability problems arise due to bubbles in the firing chamber. These bubbles can induce droplet trajectory errors, or can cause a nozzle to fail completely. Bubbles form during a pause in printing for a particular nozzle. The sensitivity of a particular inkjet system to bubbles is highly dependent on the ink formulation, the geometry of the nozzle and firing chamber, and temperature.
Therefore, what is needed is a method for preventing or curing reliability problems caused by formation of bubbles in the firing chamber caused by nozzle inactivity.
A method for an inkjet printhead assembly having ink ejection elements energizable by an electrical pulse to improve inkjet firing reliability by using high energy during the droplet ejection for nozzles that have been inactive and uncapped for a period of time or are printing at low image density. The invention provides a method of controlling an inkjet printhead assembly, including providing a printhead assembly having ink ejection elements energizable by an electrical pulse having a first predetermined energy; monitoring each print cartridge individually and determine the print density of the print cartridge elapsed time since each ink ejection element on the print cartridge has been fired; comparing the elapsed time for each ink ejection element on the print cartridge with a predetermined maximum amount of time for the print cartridge; and initiating high energy spitting over the spittoon for the print cartridge if the predetermined maximum amount of time has been exceeded for one of the ink ejection elements on the print cartridge.